1. Field of the Invention
This invention relates to novel mutants of the first Kunitz domain (K1) of the human lipoprotein-associated coagulation inhibitor LACI, which inhibit plasmin. The invention also relates to other modified Kunitz domains that inhibit plasmin and to other plasmin inhibitors.
2. Description of the Background Art
The agent mainly responsible for fibrinolysis is plasmin, the activated form of plasminogen. Many substances can activate plasminogen, including activated Hageman factor, streptokinase, urokinase (uPA), tissue-type plasminogen activator (tPA), and plasma kallikrein (pKA). pKA is both an activator of the zymogen form of urokinase and a direct plasminogen activator.
Plasmin is undetectable in normal circulating blood, but plasminogen, the zymogen, is present at about 3 μM. An additional, unmeasured amount of plasminogen is bound to fibrin and other components of the extracellular matrix and cell surfaces. Normal blood contains the physiological inhibitor of plasmin, α2-plasmin inhibitor (α2-PI), at about 2 μM. Plasmin and α2-PI form a 1:1 complex. Matrix or cell bound-plasmin is relatively inaccessible to inhibition by α2-PI. Thus, activation of plasmin can exceed the neutralizing capacity of α2-PI causing a profibrinolytic state.
Plasmin, once formed:
i. degrades fibrin clots, sometimes prematurely;
ii. digests fibrinogen (the building material of clots) impairing hemostasis by causing formation of friable, easily lysed clots from the degradation products, and inhibition of platelet adhesion/aggregation by the fibrinogen degradation products;
iii. interacts directly with platelets to cleave glycoproteins Ib and IIb/IIIa preventing adhesion to injured endothelium in areas of high shear blood flow and impairing the aggregation response needed for platelet plug formation (ADEL86);
iv. proteolytically inactivates enzymes in the extrinsic coagulation pathway further promoting a prolytic state.
Robbins (ROBB87) reviewed the plasminogen-plasmin system in detail. ROBB87 and references cited therein are hereby incorporated by reference.
Fibrinolysis and Fibrinogenolysis
Inappropriate fibrinolysis and fibrinogenolysis leading to excessive bleeding is a frequent complication of surgical procedures that require extracorporeal circulation, such as cardiopulmonary bypass, and is also encountered in thrombolytic therapy and organ transplantation, particularly liver. Other clinical conditions characterized by high incidence of bleeding diathesis include liver cirrhosis, amyloidosis, acute promyelocytic leukemia, and solid tumors. Restoration of hemostasis requires infusion of plasma and/or plasma products, which risks immunological reaction and exposure to pathogens, e.g. hepatitis virus and HIV.
Very high blood loss can resist resolution even with massive infusion. When judged life-threatening, the hemorrhage is treated with antifibrinolytics such as e-amino caproic acid (See HOOV93) (EACA), tranexamic acid, or aprotinin (NEUH89). Aprotinin is also known as Trasylol™ and as Bovine Pancreatic Trypsin Inhibitor (BPTI). Hereinafter, aprotinin will be referred to as “BPTI”. EACA and tranexamic acid only prevent plasmin from binding fibrin by binding the kringles, thus leaving plasmin as a free protease in plasma. BPTI is a direct inhibitor of plasmin and is the most effective of these agents. Due to the potential for thrombotic complications, renal toxicity and, in the case of BPTI, immunogenicity, these agents are used with caution and usually reserved as a “last resort” (PUTT89). All three of the antifibrinolytic agents lack target specificity and affinity and interact with tissues and organs through uncharacterized metabolic pathways. The large doses required due to low affinity, side effects due to lack of specificity and potential for immune reaction and organ/tissue toxicity augment against use of these antifibrinolytics prophylactically to prevent bleeding or as a routine postoperative therapy to avoid or reduce transfusion therapy. Thus, there is a need for a safe antifibrinolytic. The essential attributes of such an agent are:                i. Neutralization of relevant target fibrinolytic enzyme(s);        ii. High affinity binding to target enzymes to minimize dose;        iii. High specificity for target, to reduce side effects; and        iv. High degree of similarity to human protein to minimize potential immunogenicity and organ/tissue toxicity.All of the fibrinolytic enzymes that are candidate targets for inhibition by an efficacious antifibrinolytic are chymotrypin-homologous serine proteases.Excessive Bleeding        
Excessive bleeding can result from deficient coagulation activity, elevated fibrinolytic activity, or a combination of the two conditions. In most bleeding diatheses one must control the activity of plasmin The clinically beneficial effect of BPTI in reducing blood loss is thought to result from its inhibition of plasmin (KD˜0.3 nM) or of plasma kallikrein (KD˜100 nM) or both enzymes.
GARD93 reviews currently-used thrombolytics, saying that, although thrombolytic agents (e.g. tPA) do open blood vessels, excessive bleeding is a serious safety issue. Although tPA and streptokinase have short plasma half lives, the plasmin they activate remains in the system for a long time and, as stated, the system is potentially deficient in plasmin inhibitors. Thus, excessive activation of plasminogen can lead to a dangerous inability to clot and injurious or fatal hemorrhage. A potent, highly specific plasmin inhibitor would be useful in such cases.
BPTI is a potent plasmin inhibitor; it has been found, however, that it is sufficiently antigenic that second uses require skin testing. Furthermore, the doses of BPTI required to control bleeding are quite high and the mechanism of action is not clear. Some say that BPTI acts on plasmin while others say that it acts by inhabiting plasma kallikrein. FRAE89 reports that doses of about 840 mg of BPTI to 80 open-heart surgery patients reduced blood loss by almost half and the mean amount transfused was decreased by 74%. Miles Inc. has recently introduced Trasylol in USA for reduction of bleeding in surgery (See Miles product brochure on Trasylol, which is hereby incorporated by reference.) LOHM93 suggests that plasmin inhibitors may be useful in controlling bleeding in surgery of the eye. SHER89 reports that BPTI may be useful in limiting bleeding in colonic surgery.
A plasmin inhibitor that is approximately as potent as BPTI or more potent but that is almost identical to a human protein domain offers similar therapeutic potential but poses less potential for antigenicity.
Angiogenesis:
Plasmin is the key enzyme in angiogenesis. OREI94 reports that a 38 kDa fragment of plasmin (lacking the catalytic domain) is a potent inhibitor of metastasis, indicating that inhibition of plasmin could be useful in blocking metastasis of tumors (FIDL94). See also ELLI92. ELLI92, OREI94 and FIDL94 and the references cited there are hereby incorporated by reference.
Plasmin
Plasmin is a serine protease derived from plasminogen. The catalytic domain of plasmin (or “CatDom”) cuts peptide bonds, particularly after arginine residues and to a lesser extent after lysines and is highly homologous to trypsin, chymotrypsin, kallikrein, and many other serine proteases. Most of the specificity of plasmin derives from the kringles' binding of fibrin (LUCA83, VARA83, VARA84). On activation, the bond between ARG561-Val562 is cut, allowing the newly free amino terminus to form a salt bridge. The kringles remain, nevertheless, attached to the CatDom through two disulfides (COLM87, ROBB87).
BPTI has been reported to inhibit plasmin with KD of about 300 pM (SCHN86). AUERS8 reports that BPTI(R15) has Ki for plasmin of about 13 nM, suggesting that R15 is substantially worse than K15 for plasmin binding. SCHN86 reports that BPTI in which the residues C14 and C38 have been converted to Alanine has Ki for plasmin of about 4.5 nM. KIDO88 reports that APP-I has K; for plasmin of about 75 pM (7.5×10−10 M), the most potent inhibitor of human plasmin reported so far. DENN94a reports, however, that APP-I inhibits plasmin with Ki=225 nM (2.25×10−7 M). Our second and third library were designed under the assumption that APP-I is a potent plasmin binder. The selection process did not select APP-I residues at most locations and the report of DENN94a explains why this happened.
With recombinant DNA techniques, it is possible to obtain a novel protein by expressing a mutated gene encoding a mutant of the native protein gene. Several strategies for picking mutations are known. In one strategy, some residues are kept constant, others are randomly mutated, and still others are mutated in a predetermined manner. This is called “variegation” and is defined in Ladner et al. U.S. Pat. No. 5,223,409, which is incorporated by reference.
DENN94a and DENN94b report selections of Kunitz domains based on APP-I for binding to the complex of Tissue Factor with Factor VIIa. They did not use LACI-K1 as parental and did not use plasmin as a target. The highest affinity binder they obtained had KD for their target of about 2 nM. Our first-round selectants have affinity in this range, but our second round selectants are about 25-fold better than this.
Proteins taken from a particular species are assumed to be less likely to cause an immune response when injected into individuals of that species. Murine antibodies are highly antigenic in humans. “Chimeric” antibodies having human constant domains and murine variable domains are decidedly less antigenic. So called “humanized” antibodies have human constant domains and variable domains in which the CDRs are taken from murine antibodies while the framework of the variable domains are of human origin. “Humanized” antibodies are much less antigenic than are “chimeric” antibodies. In a “humanized” antibody, fifty to sixty residues of the protein are of non-human origin. The proteins of this invention comprise, in most cases, only about sixty amino acids and usually there are ten or fewer differences between the engineered protein and the parental protein. Although humans do develop antibodies even to human proteins, such as human insulin, such antibodies tend to bind weakly and the often do not prevent the injected protein from displaying its intended biological function. Using a protein from the species to be treated does not guarantee that there will be no immune response. Nevertheless, picking a protein very close in sequence to a human protein greatly reduces the risk of strong immune response in humans.
Kunitz domains are highly stable and can be produced efficiently in yeast or other host organisms. At least ten human Kunitz domains have been reported. Although APP-I was thought at one time to be a potent plasmin inhibitor, there are, actually, no human Kunitz domains that inhibit plasmin as well as does BPTI. Thus, it is a goal of the present invention to provide sequences of Kunitz domain that are both potent inhibitors of plasmin and close in sequence to human Kunitz domains.
The use of site-specific mutagenesis, whether nonrandom or random to obtain mutant binding proteins of improved activity is known in the art, but success is not assured.